186 JOHN LEWIS BREMER 



mon (fgs. 8 and 10). Also anastomoses between paired dorsal 

 segmental arteries are frequently found, and account for their 

 later single trunk without the supposition that the two vessels 

 fuse. The paired aortae, in the cervical region, may even send 

 vessels toward the median line to form a plexus, especially well 

 developed in the cat. 



Through such a plexus any one of several paths may be chosen 

 for a permanent vessel, and until the actual degeneration of the 

 roots has taken place such a path may be moved from one posi- 

 tion on the aortic wall to another. The meson ephric arteries 

 of adult selachians are said by Dohrn (11) to arise normally 

 from the lumbar arteries; a glance at f gure 4 will show how this 

 could come about. Even the extreme case of Bdellostoma, in 

 which the adult mesenteric arteries arise from the dorsal wall 

 of the aorta in common with the dorsal segmental arteries, is 

 explicable. 



With the general growth of the area around the aorta, or with 

 the special lengthening of any of its main branches, the anas- 

 tomotic vessels of the plexus may come to lie outside the aortic 

 wall, and the regular meshes of the net become thus distorted. 



Fig. 1 Sheep embryo, 5.6 mm. H.E.C. No. 1332, Sect. 220-235. Recon- 

 struction of part of the abdominal aorta to show right mesonephric arteries from 

 left aorta, rich., notochord; wcs'., root of mesentery; /».r;r/., mesonephric arteries. 

 X 100 diam. 



Figs. 3 and 4. Pig embryo, 24.0 mm., H.E.C. No. 1846, Sect. 1585-1610. Lat- 

 eral view and semi-diagrammatic transverse section of a reconstruction of a por- 

 tion of the abdominal aorta and periaortic net. The common trunk of the dor- 

 sal segmental arteries, d.cirt., is connected within the aortic wall with a 

 mesonephric artery, m.art. An intervening branch, j, from the aortic cavity has 

 lost its connection. The lateral branches of the plexus suggest at least one more 

 intervening branch to complete the net (cf. fig. 2). X 40 diam. 



Fig. 5 Pig embryo, 24.0 mm., H.E.C. No. 62, Sect. 1375-1525. Reconstruc- 

 tion of the dorsal aspect of abdominal aorta, to show anastomoses wathin and 

 without the aortic wall between the dorsal segmental arteries and lateral aortic 

 branches, one of which, A-, arises as an intercalated dorsal vessel, runs to a lat- 

 eral position, and emerges laterally to reach the kidney. There are two sets of 

 dorsal branches per segment, d.ari., dorsal segmental vessels, some with com- 

 mon trunk, some paired though connected. X 40 diam. 



Fig. 10 Man, 16.4 mm., H.E.C. No. 1707, Sect. 774-858. Explanation in 

 text, coe.a., coeliac axis; s.m.art., superior mesenteric artery; sw/^r.o?-/., supra- 

 renal artery; r.art., renal artery. X 60 diam. 



